A radio frequency (RF) modulator is a device that receives a baseband signal and generates a modulated signal based thereon. For example, an RF modulator may be used to receive a video and/or audio signal from an input source such as a DVD player, VCR, Camcorder, digital audio players, a media player, or other suitable video and/or audio signal source and generate a modulated signal that can be fed to a video and/or audio output device such as an analog television or other suitable device. Some input devices include an RF modulator allowing the output of the device to be fed directly into a television. However, newer input video devices do not typically include an RF modulator. These newer devices typically generate composite, component, S-Video, or other format for signals that are fed directly into newer televisions that include an input capable of receiving these signals.
In order to use a newer input device with a legacy output device that does not include inputs for composite, component, S-Video, or other format, an RF modulator may be operatively coupled within or between the newer input device and the legacy output device. When the output device, such as an analog television, needs to receive a frequency modulated (FM) signal, the RF modulator typically includes at least one variable oscillator capable of modulating the output signal in various frequency bands or channels. These variable oscillators are typically voltage controlled and are often referred to as voltage controlled oscillators (VCOs).
A tuner typically communicates with the VCO to adjust the oscillations to a predetermined frequency. The tuner may include level shifting circuits that provide control signals to varactors that are part of the VCO. When adjusting the oscillations of the VCO by controlling the varactors, such as when tuning from one television channel to another, the level shifting circuits in the tuner require more power and thus more current than when the tuner is fixed on a channel. However, when supplying a higher current to the level shifting circuits, noise may be induced in the RF modulator. The noise may adversely interfere with the modulated signal and consequently reduce the quality of the video and/or audio output.
In one method, an integrated circuit (IC) that includes a single mode passive voltage supply is used to supply power to the tuner. The IC typically includes an internal voltage divider circuit that is operatively coupled to a large external (e.g., off chip) capacitor via pins. The external capacitor is used to supply the additional current needed when the tuner is adjusting the oscillations of the VCO. Although this method works, it requires a large external capacitor coupled to the IC and additional pin count, which increases the size and cost of the voltage supply.
What is desired is an improved voltage supply circuit.